1. Field of the Invention
This invention relates to a balancing device for vertically moving dental arm assembly used for a dental light, dental tray, X-ray source, or the like.
2. Prior Art
A conventional balancing device for a dental light fixed to a moving arm includes a spring as shown in FIGS. 1 (A) and 1 (B). FIG. 1 (A) is a partially cutaway plane view of the balancing device, and FIG. 1 (B) is a sectional view taken along the line Y--Y in FIG. 1 (A). Referring to these figures, a light arm b is connected to a hanger arm through a spring member c so that it can move vertically. With this device, it is impossible to balance the arms a and b by using the restoration force of the spring member c. A wing nut c.sub.1 is tightened or loosened to securely position the light arm b. The light arm b is vertically fixed by the friction caused by the tightened wing nut c.sub.1. This tightening/loosening, however, is troublesome because the lamp head of the dental light must be moved frequently and positioned accurately. In addition, the movement of the light arm be is not smooth. These are a mental burden to the operator.
A parallel rod type (not shown) is also used for this kind of balancing device. The device of this type has a very high operability, since it is balanced mechanically. However, it is complicated, expensive and larg. If the dental light including this balancing device is placed over the operator and patient, it causes excessive mental pressure and uncomfortableness.
Furthermore, the dental arm swinging device, Japanese Utility Model laid-open No. 57-185114, proposed by the applicant of the present invention, is very useful for a swinging movement with a relatively small rotation moment. However, it is not suited for a balancing device used for a large arm which needs a rotation moment range of 50-80 kgcm.